Vacationers to Yosemite National Park are getting quite a scare this week. On Tuesday, 1,700 visitors were informed by officials that they may have been exposed to hantavirus and were at risk of developing hantavirus pulmonary syndrome (HPS), a rare rodent-borne illness that has already claimed the lives of two national park visitors.

Around 1700 people were alerted by email that those who stayed in the “Signature Tent Cabins” in Yosemite’s Curry Village from mid-June through the end of August may be at risk for the disease that causes flu-like symptoms including fever, aches and chills. The virus, which is transmitted by rodents such as deer mice, is spread to people who inhale dust particles that have come in contact with urine, saliva and feces of infected animals. So far, three cases of the disease have been confirmed and health officials are investigating a fourth probable case.

A man from Alameda County in the San Francisco Bay area was the first among this cluster of cases to die of HPS, and the second victim is also a California resident.

The Associated Press reports that since thousands of people visit the park every month, it’s impossible to track everyone who passed through Curry Village. The popular site is located at the base of the 3,000-foot Glacier Point and houses 408 tent cabins. Ninety-one are considered signature cabins where the four infected visitors stayed.

“We don’t fully know what’s going on with this situation,” says Yosemite National Park ranger Jana McCabe. “We don’t know if something has changed in the environment, but we do know there are ways people can protect themselves. We are asking people not to sweep up mouse droppings if they see evidence of them, but to contact park staff. We are trained on how to respond to it.”

The hantavirus circulating in North America was first identified in 1993 by scientists at the U. S. Centers for Disease Control and Prevention (CDC) after an outbreak of a mysterious respiratory illness killed 31 otherwise healthy young people living in the “Four Corners” area bordering Arizona, Colorado, New Mexico and Utah. The virus hunters traced the deaths to exposure to infected deer mice and identified a new form of the already described hantavirus. Since then, 556 cases of the disease have been reported in the U.S., with a mortality rate of 36%.

McCabe says the park has always followed the necessary precautions for keeping rodents out of living areas, but given that they are such a prevalent part of the ecosystem, it’s not always possible to protect campers or hikers.

“The health of our visitors is our paramount concern and we are making every effort to notify and inform our visitors of any potential illness,” said Don Neubacher, Yosemite National Park Superintendent in a statement.

If you’ve recently visited Yosemite, or are concerned that you might have been exposed to the virus, here’s what you should know to protect yourself.

1. Where is hantavirus found?
The CDC says the disease occurs sporadically, typically in rural areas with lots of forests, fields and farms where rodents reside. Barns, outbuildings and sheds in such areas could be spots where people can come in contact with infected animals. Of the 556 cases of the disease that have been reported since the virus was identified, about three fourths of those infected lived in rural areas. Cases have been reported in 34 states and large outbreaks still remain very rare.

“If you’re camping in the back country it’s not likely you will come in contact with it because you’re not in an enclosed space,” says McCabe. “We know the virus does not survive long in sunlight. It’s more commonly found under the kitchen sink. If you’re going to place that’s been closed up for a long time, be sure to open it up and let it air out.”

2. How are people infected with hantavirus?
The primary route of exposure is via rodents, so infestations should be eliminated. People pick up the virus through airborne transmission, inhaling dust infected with the microbe in the urine, feces and saliva of rodents.

It’s important to note that so far, there are no reported cases of people transmitting the virus to other people, but the CDC cites some other ways humans can come in contact with the disease:

If an infected rodent bites someone, the virus could spread, although this is still rare.

People may contract the virus if they touch something contaminated with rodent urine, droppings, or saliva, and then touch their nose or mouth.

It’s possible people can become sick if they eat food contaminated by urine, droppings, or saliva from an infected rodent.

Opening and cleaning previously unused buildings.

Work-related exposure such as those working under homes or in crawl spaces.

3. What are the symptoms of HPS?
Since cases of HPS are extremely uncommon, officials are unsure of the exact incubation period for the disease, but they estimate that people can show signs of illness one to five weeks after exposure. Early symptoms include fatigue, fever and muscle aches in the thighs, hips, back, and sometimes shoulders. The CDC says about half of HPS victims will experience headaches, dizziness, chills, nausea, vomiting, diarrhea and abdominal pain. As the disease progresses, symptoms may expand to include coughing and shortness of breath.

4. How can I prevent infection?
Yosemite National Park officials say the following precautions can help protect people in the wilderness or other areas that harbor mice:

Avoid areas, especially indoors, where wild rodents are likely to have been present.

Keep food in tightly sealed containers so rodents can’t get to them.

Keep rodents out of buildings by removing stacked wood, rubbish piles, and discarded junk from around homes and seal any holes where rodents could enter.

When cleaning a sleeping or living area, open windows to air out the areas for at least two hours before entering. Try not to stir up dust. Wear plastic gloves and spray areas contaminated with rodent droppings and urine with a 10% bleach solution or other household disinfectants and wait at least 15 minutes before cleaning the area. Place the waste in double plastic bags, each tightly sealed, and discard in the trash. Wash hands thoroughly afterward.

Do not touch or handle live rodents and wear gloves when handling dead rodents. Spray dead rodents with a disinfectant and dispose of in the same way as droppings. Wash hands thoroughly after handling dead rodents.

If there are large numbers of rodents in a home or other buildings, contact a pest control service to remove them.

5. How is HPS treated?
Park officials say early medical attention is critical for individuals who contract HPS or have symptoms of the disease. Currently, there is no treatment, cure, or vaccine for HPS, but early medical care can help to prevent life-threatening respiratory problems. The CDC says in intensive care, patients are intubated and given oxygen therapy to aid them during any respiratory distress. The earlier patients are brought into care, the more likely the treatment will be effective.